Fix minor problems found by static checking.

Fix minor problems found by static checking.

* coding.c (encode_inhibit_flag, inhibit_flag): New functions.
Redo the latter's body to sidestep GCC parenthesization warnings.
(setup_coding_system, detect_coding, detect_coding_system): Use them.
* coding.c (detect_coding, detect_coding_system):
* coding.h (struct undecided_spec):
Use bool for boolean.
* image.c (QCmax_width, QCmax_height): Now static.
* xdisp.c (Fmove_point_visually): Remove unused local.